How to Extract Integers from User Input using Regex in Python

Processing user input often involves cleaning unstructured text to extract specific data types, such as integers. While you could iterate through characters manually, Python's re (Regular Expression) module provides a much more efficient and readable solution.

This guide explains how to use regular expressions to find, extract, and format integers from any given string.

Understanding the Regex Pattern

To extract numbers from mixed text (e.g., "Item 12 costs $50"), we need a pattern that identifies digits.

• \d: Matches any decimal digit (0-9).
• +: Quantifier meaning "one or more" of the preceding element.

Therefore, the pattern r"\d+" matches sequences of one or more digits.

note

Using \d alone would match 1 and 2 separately in the number 12. Using \d+ ensures 12 is treated as a single integer.

Step 1: Setting Up the Script

Regular expressions are not loaded by default. You must import the standard re module.

Create a file named find_int.py and add the import statement.

import re
# No external installation required; 're' is part of the standard library.

Step 2: Implementing Extraction Logic

We use re.findall() to locate all non-overlapping matches of the pattern in the string. It returns a list of strings.

import re

text = "Order ID: 5592, Qty: 3"

# ⛔️ Naive Approach: Iterating and checking isdigit()
# This is verbose and hard to handle multi-digit numbers (like 5592) correctly
digits = []
temp = ""
for char in text:
    if char.isdigit():
        temp += char
    else:
        if temp: digits.append(temp)
        temp = ""

# ✅ Correct: Using Regex
# Returns a list: ['5592', '3']
integers = re.findall(r"\d+", text)

Step 3: Handling Input and Output

The goal is to return a space-separated string of numbers found in the input.

def extract_integers(user_input):
    # Find all sequences of digits
    integers_list = re.findall(r"\d+", user_input)
    
    # Join list elements into a single string separated by spaces
    return " ".join(integers_list)

Complete Code Solution

Here is the complete find_int.py script. It accepts input from the command line (standard input) and prints the extracted numbers.

import re

def extract_integers(user_input):
    """
    Finds all integers in a string and returns them as a space-separated string.
    """
    # Pattern explanation:
    # \d  -> Matches digits 0-9
    # +   -> Matches 1 or more repetitions
    integers = re.findall(r"\d+", user_input)
    
    # Join the list ['1', '2'] into "1 2"
    return " ".join(integers)

if __name__ == "__main__":
    try:
        # Get input from user (waits for typing in console)
        user_text = input()
        
        # Process and print
        result = extract_integers(user_text)
        print(result)
    except EOFError:
        pass # Handle cases where input might be empty/closed abruptly

Testing the Script

Test Case 1 Input: a1b2c3d4

$ python find_int.py
a1b2c3d4

Output:

1 2 3 4

Test Case 2 Input: 12 3ad5

$ python find_int.py
12 3ad5

Output:

12 3 5

tip

re.findall returns strings (['12', '3', '5']). If you need to perform mathematical operations on these results later, remember to convert them using int() or float().

Conclusion

Regular expressions are the most robust tool for text parsing in Python.

1. Import re to access regex tools.
2. Use r"\d+" to match whole integers (sequences of digits).
3. Use re.findall() to extract all occurrences into a list.
4. Use join() to format the output list back into a string.